This invention relates to a reflecting mirror having a variable-shape reflecting surface, and more particularly a variable-curvature reflecting mirror for a laser having a variable-curvature, spherical reflecting surface. This variable-curvature reflecting mirror is especially suitable as a beam propagating reflecting mirror for a laser machining device and as a reflecting mirror for use in an oscillator. By controlling the curvature of the mirror, it is possible to adjust the beam diameter at the final beam focusing point, focal point and the beam mode.
A conventional reflecting mirror having a variable-shape reflecting surface is introduced in a publication by SPIE (The Society of Photo-optical Instrumentation Engineers), vol 1543, on page 36. This is a technique concerning a reflecting mirror of an astronomical telescope. The variable-shape mirror compensates for any turbulence of wavefronts of the incoming beam. In this case, the reflecting surface is deformed to create a desired shape.
In contrast, in a variable-curvature reflecting mirror which is used as a beam propagating reflecting mirror for a laser machining device and a reflecting mirror for use in an oscillator in recent years, the curvature of the spherical mirror is simply controlled instead of creating a desired shape. This variable-curvature mirror is simple, compact and cheap compared with the variable-shape mirror.
In a variable-curvature mirror to which this invention relates, means for deforming the reflecting mirror under pressure include the type in which static pressure is applied to a wide area of the back of the reflecting mirror (unexamined Japanese patent publication 2-231779) and the type in which pressure is directly applied to a limited area of the back of the mirror by e.g. an actuator (direct pressure herein used includes a force transmitted through several fixed parts). This invention is applicable to either type, but for high-accuracy deformation control, the direct-pressure type is preferable to the static pressure type (because in the latter type, it is difficult to seal a static-pressure-producing, liquid pressure medium in a chamber formed in the back of the reflecting mirror for a long time without leakage). The following description is therefore made for the direct-pressure type.
Conventional, direct-pressure type, curvature-variable mirrors are disclosed in e.g. unexamined Japanese patent publications 8-39282 and 9-293915.
The most important characteristics required for a reflecting mirror for use in a laser is the shape accuracy of the reflecting mirror (that is, a deviation from an ideal plane in the case of a plane mirror, and a deviation from an ideal sphere in the case of a spherical mirror). If there is a deviation of 1 .mu.m or over in the shape of the reflecting surface, wavefronts of the reflected beam tend to be distorted, and thus the beam condensability tends to drop. This makes impossible high-intensity condensation, which is the main feature of laser beams. Thus, it is necessary to reduce a deviation in shape to less than 1 .mu.m, ideally to less than 0.5 .mu.m.
In order to achieve such high shape accuracy of a variable-curvature mirror over the entire curvature range, the following three requirements have to be met.
a) Original shape accuracy of the reflecting surface is not worsened but maintained when the curvature is changed. PA1 b) The reflecting mirror support member does not contribute to the worsening of the shape accuracy of the reflecting mirror. PA1 c) The reflecting mirror is less susceptible to such damage as scratches and permanent deformation.
None of conventional mirrors meet all these requirements. For example, unexamined Japanese patent publication 8-39282 discloses a technique in which the thickness of the reflecting mirror is changed, i.e. reduced toward the outer edge to increase the sphere-maintainable range and thus to satisfy the requirement a). In the technique of this publication, the reflecting mirror is bent by resiliently deforming the boundary portion with an outer cylindrical portion with the reflecting mirror completely fixed, so that a large deformation pressure is required and it is difficult to deform the mirror to an ideal shape up to its outer peripheral portion. Also, the reflecting mirror tends to be fatigued with time at its support point of deformation.
On the other hand, in unexamined Japanese patent publication 9-293915, for easy deformation to the outer peripheral portion, the reflecting mirror has its outer edge in line contact so that no restriction to pivotal movement occurs when deformed. But since the mirror is pressed by O-rings from both sides to prevent restriction to pivotal movement at the outer peripheral portion, the reflecting mirror tends to translate in the direction of pressure application. This results in the loss of stroke of the actuator.
An object of this invention is to maintain high shape accuracy by deforming the reflecting mirror under a low load while preventing stroke loss of the actuator.